While pipelines, for example, for natural gas and oil, are relatively safe systems for transporting such products, nevertheless such pipelines have been known to fail often with tragic economic and human loss. From time to time such failures are the direct result of human activity in the vicinity of the pipeline, for example, when earth moving equipment at a construction site accidentally strikes a section of a pipeline. Not so commonly known is the fact that operational pipelines have failed from other than such human causes, always with dramatic suddenness and often with tragic and serious loss to life and property. For example, a 12 inch natural gas pipeline broke unexpectedly in the main business district of Malton, Ontario, Canada, on Oct. 25, 1969. One person was killed and numerous others were injured when this pipeline burst and gas flowing therefrom burst into flames. The flames were out of control for several hours and caused extensive damage to Malton's business section. In another example, at 3:40 p.m. on Sept. 9, 1969, a 14 inch pipeline carrying natural gas at a pressure of more than 780 p.s.i.g. ruptured in a residential subdivision 31/4 miles north of Houston, Tex. It was determined that the probable cause of the accident was the fracturing of a length of pipe along a weak zone. The fracture travelled 46 feet. One hundred and six houses were damaged, property damage was estimated at $500,000. and nine people were injured, two seriously. Another time, during the 1960's, in what has been called the "Great Lakes Pipeline Failure", at least 1 mile of natural gas transmisson line pipe south of Lake Superior suddenly burst, again with substantial economic loss. This 168-mile 36-inch diameter natural gas pipeline operated by the Great Lakes Gas Transmisson Company extends from Emerson, Manitoba, to Sarnia, Ontario, passing through the states of Minnesota, Wisconsin and Michigan.
It has been determined through extensive and costly research on pipelines, that some of these failures have been caused by pipeline fractures which have developed from cracks which have appeared in operating pipelines. Cracks have formed when gases, for example, are transported under pressure in such pipelines and such cracks have travelled to anywhere from a few feet to 1 mile or more along the pipeline. Where the pipeline transported liquids, this distance has been much less. Extensive tests have been carried out in the United States, sponsored by the American Gas Association to determine the nature and causes of, and solutions for such failures. Results of this research have been reported by the American Gas Association in "Symposium on Line Pipe Research (a progress report on Project NG-18 being carried out by the Pipeline Research Committee at Battelle Memorial Institute)" and "Fourth Symposium on Line Pipe Research". As a result of this research, it has been concluded that the initiation and propagation of cracks in operational pipelines are significantly reduced by increasing the yield strength and toughness of the pipelines. This research has also determined that as the temperature of the pipeline decreases, the initiation of fractures and their rate of propagation increases. As well, it has been determined that the danger of such fractures occurring in operational pipelines increased as the diameter of the pipeline increases and as the load pressure of the pipeline increases. Thus, the dangers apparent with the 48 and 60 inch diameter pipelines presently being constructed and contemplated for transmisson of gas and oil in the arctic can be readily understood; such pipelines, because of their size and because of their operational temperatures are much more prone to the initiation of these fractures. When the load pressure of a pipeline is high, for example in transmission line pipe, or is increased, for example where increased flow of gas through a pipeline is required, then "hoop stress", acting circumferentially throughout the pipeline, is an important factor. When hoop stress becomes greater than yield strength, then the danger of fracture of the pipeline becomes serious. Thus, a minimum yield strength is prescribed by Government standards for a given pipe to be used in the pipeline, which yield strength is higher than would be required for the highest pressure normally exerted by a fluid in the pipeline.
It is obvious from the above comments relating to research which has been done on pipeline failures that, to reduce the likelihood of such failures due to cracks and to reduce the rate of crack propagation, the yield strength and toughness of unit pipes for a pipeline should be increased. It is known that to increase the strength and toughness of steel pipes, the pipe may be heated to above its austenitizing temperature, quenched and then tempered to bring the steel to the desired degree of strength. Practical difficulties, however, are encountered with such a method in that large furnaces and quenching baths must be constructed so that the entire pipe may be treated. These difficulties are compounded with larger pipes, for example of the 48 inch to 60 inch diameter, so that furnaces and baths for such sizes of pipes are at the present time not commercially available. With heat treatment of such larger pipes, distortion also becomes a serious problem. As well, energy requirements for heating and tempering the entire pipe unit are very great.
In addition, it has been found as a practical matter that steel which has been strengthened to over 60,000 or 70,000 p.s.i. is extremely difficult to weld throughout its length during manufacture of pipe or, in the form of pipes, end to end in the field. Normal field welding of pipes does not require heating of the ends of the pipe beforehand or heating of the weld zone after the pipe has been welded. To weld such high strength steel, the weld zone must be pre-heated to 1.degree. to 200.degree. F. and post-heated after welding. As well, this method requires special welding wire or welding rods and takes a long period of time for each weld. Also, specially trained personnel are needed. One such pipeline, an experimental natural gas pipeline, 1200 feet in length and 36 inches in diameter was constructed during the 1960's by the Columbia Gas System, Columbus, Ohio, U.S.A. Thus, where the entire length of pipe is heat treated to a yield strength in excess of 70,000 p.s.i., in order to minimize crack initiation and propagation in transmission line pipe, the ends of the pipes so treated are either incapable of being successfully welded by any known acceptable conventional process or may be welded with great difficulty.
Other methods of increasing the overall yield strength and toughness of units of pipe in order to resist the formation and propagation of such pipeline fractures might be considered. One method involves winding high strength steel wire around the pipe to be strengthened. Of course this method is very expensive and time consuming, requiring special reinforcing wire and special machinery for wrapping the wire around the pipe. Another suggested method would require the welding of steel bands about the pipe in order to reinforce it. Where the pipe unit has been previously heat treated to a desired degree of strength and toughness, the heat from the welding process may well adversely affect this pipeline by reducing its strength. As well, this process again is expensive and cumbersome.
It is an object of the present invention to provide a steel pipe or tubing having greater resistance to crack initiation or propagation than steel pipe or tubing presently known, which pipe or tubing may be welded in the field in a conventional manner without the difficulties experienced with welding steel pipe or tubing having high yield strength. It is a further object of this invention to provide a more economical and more efficient method of strengthening and toughening steel pipes and tubing in order to resist such crack initiation and propagation.